Process for stabilizing gasoline



Patented Oct. 10, 1950 2,525,152 PROCESS roa STABILIZING GASOLINE Barney B. Strickland, Westfleld, and Emil H. Lewis, Union, N. 1., minors to Standard Oil Development Company, a corporation of Dela- Application September 17, 1947, Serial No. 774,656

2 Claims.

' mixtures boiling in the gasoline boiling range and produced by cracking processes. In its specific adaptation the invention is particularly concerned with the stabilization. of hydrocarbons of the aforesaid type which have been produced by catalytic cracking operations.

We have discovered that olefins and diolefins normally present in hydrocarbons boiling in the motor fuel boiling range, particularly as produced in a catalytic cracking operation, are oxidized to form peroxides. As a result, the presence of these compounds normally causes excessive gum formation in the hydrocarbon product. In order to remove this gum, additional processing steps, such as acid treating or distillation, are normally required. It is an object of this invention to avoid such processing steps by stabilizing the gasoline to prevent the formation of excessive gum.

In accordance with our invention, hydrocarbons boilin in the motor fuel boiling range, particularly as produced in a catalytic cracking operation, are treated with aqueous caustic solutions in the absence of oxygen. By means of this treatment such hydrocarbon mixtures are stabilized so as to substantially prevent the formation of gum.

It is known in the art to produce hydrocarbons boiling in the motor fuel boiling range by various distillation, reforming, and cracking procedures. No attempt will be made to outline or discuss these processes since they are well known. The products from such processes generally contain varying amounts of aliphatic and aromatic mercaptans. In general, thermally cracked stocks may contain minor amounts of aromatic mercaptans, such as thiocresols for example. However, in the case of catalytically cracked gasolines, it is found that these gasolines in general have a rather small amount of aliphatic mercaptans but a relatively large amount of aromatic mercaptans. This is particularly the situation when high sulfur stocks are employed. For example, when a West Texas gas oil, having a boiling range of about 400 F. to 700 F. and having a sulfur content of about. 1.5%. catalytically cracked to a to yield of 400 F. and point gasoline, the gasoline will have about a 30 copper number; that is, 30 milligrams of mercaptan sulfur per milliliters of gasoline. This figure corresponds to approximately 0.035% sulfur as aromatic mercaptans. Or again, when a mixed gas oil of about 0.8% sulfur is catalytically cracked to a 50% yield of 400 F. end point gasoline, the amount of aromatic mercaptan sulfur in the gasoline is about 0.017 gram per 100 milliliters of gasoline.

As pointed out heretofore, catalytically cracked gasolines contain mercaptans, substantially all of which are aromatic in nature. We have now discovered a peculiar phenomenon with respect to catalytically cracked gasolines containing these aromatic mercaptans. We have discovered that when oxygen is present, oxidation of olefins and diolefins in the gasoline is catalyzed by the pres- TABLE I Eflect of mercaptans 1 and hydrocarbon compostsition an peroxidation and air sweetening l Mercaptans added to theoretical copper number of 30.

1 Yule, J. A. C. and Wilson, C. P. Jr. Ind. Eng. Chem. 23, 1254 (1931). I Francis, 0. K., Oil 6: Gas Journal, 36, No. ll, 99 (1937).

4 Bureau of Mines, Reports of Investigations No. 3152, November 1931.

i A. S. T. M. D38l-44.

3 The table shows experiments with two base stocks A and D. Hydrocarbon base stock A was olefinic, consisting of 75% diisobutylene and 25% The exact mechanism of the reactions involved in this phenomenon i not known. However, it

is believed that olefins and diolefins in the presdimethylbutadiene, while base stock D was parafflnic consisting of normal heptane. It will be noted from the table that the base stocks contained no peroxides and substantially no gum, both before and after air contact.

Stocks B and C consisted of the base stock A, plus suflicient mercaptans to result in a theoretical copper number of 30. Similarly stocks E and F consisted of the base stock D, plus sufficient mercaptans to produce a theoretical copper number of 30. Two mercaptans were added to each fuel base; normal heptyl mercaptan which is an aliphatic mercaptan, and parathiocresol which is an aromatic mercaptan. It will be observed from the table that addition of the altphatic mercaptan to either fuel base did not appreciably increase the gum content of the blend. The peroxide number after contacting with air as contrasted to before contacting with air, was also not increased, indicating that no oxidation took place. I

However, in noting the efiect of adding the aromatic mercaptan, parathiocresol, to the olefinic base stock A, it will be observed that this was effective in increasing the gum materially after contact with air, and also in increasing the peroxide number of the blend after air contact. This indicates oxidation of the diolefins and olefins while the decrease of the copper number indicates the destruction of the aromatic mercaptan. The resulting gum formation of 34 is excessively high and would necessitate further acid treating or redistillation. In the case where parathiocre'sol was added to normal heptane, it will be noted that the addition of this aromatic mercaptan was not effective in increasing the gum content or in changing the peroxide number of the stock.

These results show that when an aliphatic mercaptan is added to either a parafiinic or olefinic fuel base, no oxidation of the mercaptan occurs and no gum formation results from air contact.

On the other hand, when an aromatic mercaptan is added to an olefinic fuel base, oxidation of the mercaptan does occur with resulting gum formation. The fact that these results do not occur with parafiinic base fuel stock, shows the importance of the hydrocarbon composition on these reactions.

These results are further substantiated by the following data secured with hydrocarbon mixtures boiling in the motor fuel boiling range derived from catalytically cracked gasoline.

TABLE II Initial Inspec- Inspections after tions Air Contact Stock Cu Peroxide Cu Peroxide No. Number No. Number Low Pressure Distillate:

Mix A 15 3 2.6 Mix B 27 0 l 8. 9

The table shows the results of tests on two low pressure distillates, mixtures A and B. It will be observed that after air contact the copper number of these mixtures had decreased materially while the peroxide number had increased. These results show the oxidation of oleflns and diolefins in the fuel mixtures with the accompanying destruction of aromatic mercaptans.

ence of oxygen, catalyzed by the aromatic mercaptans, form peroxides which destroy the aromatic mercaptans to form gum products. The result is a gasoline containing a relatively large amount of gum which necessitates further treatment of the product.

In accordance with our invention, therefore, we propose to remove these aromatic mercaptans, particularly from catalytically cracked gasolines,

by treating these gasolines to remove the met-- captans before the cracked stock is allowed to contact oxygen. As aromatic mercaptans in the presence of olefinic hydrocarbons are very reactive with oxygen, extreme precautions must be taken in the practice of our invention to exclude all air or oxygen during the removal of the arcmatic mercaptans.

Aromatic mercaptans may be readily and substantially completely removed by caustic washing, as indicated by the following table.

TABLE III Copper Number Gasoline Afterwaslg Initially fl k g NaOH Czlatalytically Cracked Gasoline Distil- Sample A 15 2 Sample B 27 1 is present, however, the mercaptans are converted to a form not recoverable with caustic.

Our invention may be further understood by reference to the diagrammatical drawing illustrating one embodiment of the invention. Hydrocarbon feed, which for the purpose of illustration is assumed to be a high sulfur West Texas gas oil boiling in the range from about 400 to 700 F., is introduced into catalytic cracking zone In by means of line I. Reaction zone In contains a suitable fluidized catalyst and is maintained under temperature and pressure conditions designed to secure the desired cracking of the feed stock. The cracked product, comprising hydrocarbons boiling in the gasoline boiling range, is removed from reaction zone It) by means of line 2, cooled in cooling zone 3 and passed into separation zone 20. A hydrocarbon fraction having an end point of about 400 F. is removed overhead from zone 20 by means of line 4, while higher boiling constituents are removed by means of line 5. The hydrocarbon fraction removed by means of line 4 is passed through cooling zone It: and passed into separation zone 30. Uncondensed gases, comprising hydrogen sulfide, are removed overhead from zone 30 by means of line by means of line 1. In accordance with our invention, we introduce aqueous caustic by means of line 8 into the hydrocarbon stream of line 1 before this stream is allowed to contact oxygen or an oxygen-containing gas. The caustic used should preferably have been freed of oxygen by purging with an inert gas, such as light hydrocarbons, or oxygen-free nitrogen gas. After complete mixing of the caustic solution and gasoline in a suitable vessel 35, the mixture is passed into settling zone 40. The treated gasoline is allowed to separate from the aqueous caustic solution and is removed through line 9. The spent.

caustic solution is drawn off by means of line I I. It is essential that up to this point oxygen be excluded from the process. However, from this point on, the gasoline may be water washed and further processed as desired by conventional methods. The caustic used is preferably a solution of from 5 to 40 B. The amount of treating agent is preferably in the range from about 20 5% to 20% caustic per volume of oil. While the example given has referred to the treatment of gasoline with caustic solution, other hydroxides may be used. In general, any alkali metal hydroxide capable of dissolving the mercaptans may 25 6 catalytically'cracking a gas oil feed stock boiling above about 400 F. containing more than about 0.8% sulfur, fractionating the products of the said cracking step and separating a distillate fraction boiling in the gasoline boiling range substantially free of hydrogen sulfide but containing substantial quantities of mercaptans, said mercaptans consisting principally of aromatic mercaptans resulting from the cracking step'to the substantial exclusion of aliphatic mercaptans, and said distillate stock containing both olefins and dioleflns, thereafter contacting said distillate stock with an aqueous alkali hydroxide solution free of oxygen, said contact being conducted in the absence of oxygen, and thereafter separating the treated distillate stock from the spent caustic whereby a. gum stable, substantially mercaptan sulfur free gasoline is obtained. I

' 2. The. process as defined by claim 1 wherein the hydroxide solution has a strength in the range from 5' to 40 B. and is used in an amount in the range of 5% to 20% per volume of catalytically cracked gasoline.

BARNEY R. STRICKIAND. EMIL H. LEWIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATEN'IS Number Name Date 1,704,246 Halloran Mar. 5, 1929 1,752,709 Stoll Apr. 1, 1930 1,935,725 Perl Nov. 21, 1933 2,053,752

Vobach et a1. Sept. 8, 1936 

1. THE PROCESS OF PRODUCING A STABLE CRACKED GASOLINE WHICH CONSISTS OF THE FOLLOWING STEPS: CATALYTICALLY CRACKING A GAS OIL FEED STOCK BOILING ABOVE ABOUT 400*F. CONTAINING MORE THAN ABOUT 0.8% SULFUR, FRACTIONATING THE PRODUCTS OF THE SAID CRACKING STEP AND SEPARATING A DISTILLATE FRACTION BOILING IN THE GASOLINE BOILING RANGE SUBSTANTIALLY FREE OF HYDROGEN SULFIDE BUT CONTAINING SUBSTANTIAL QUANTITIES OF MERCAPTANS, SAID MERCAPTANS CONSISTING PRINCIPALLY OF AROMATIC MERCAPTANS RESULTING FROM THE CRACKING STEP TO THE SUBSTANTIAL EXCLUSION OF ALIPHATIC MERCAPTANS, AND SAID DISTILLATE STOCK CONTAINING BOTH OLEFINS AND DIOLEFINS, THEREAFTER CONTACTING SAID DISTILLATE STOCK WITH AN AQUEOUS ALKALI HYDROXIDE SOLUTION FREE OF OXYGEN, SAID CONTACT BEING CONDUCTED IN THE ABSENCE OF OXYGEN, AND THEREAFTER SEPARATING THE TREATED DISTILLATE STOCK FROM THE SPENT CAUSTIC WHEREBY A GUM STABLE, SUBSTANTIALLY MERCCAPTAN SULFUR FREE GASOLINE IS OBTAINED. 